Control of oocyte maturation in cows--biological factors.

Since bovine in vitro fertilization became possible in the early 80s, a lot of effort has been done to clarify the mechanisms of what seems more and more one of the crucial steps in this procedure, being oocyte maturation. Undoubtedly, many biological factors act together to prepare the immature oocyte for a successful development to a competent embryo after fertilization. Defects in oocyte maturation can possibly be caused by an inadequate nuclear or cytoplasmic maturation or even by a failure of both. There is a general agreement upon the fact that the origin of the oocyte can play an important role. Oocytes derived from very small follicles show a lower rate of maturation and lower blastocyst development with currently used maturation protocols. Parthenogenetic activation of small size follicle derived oocytes suggests that their poor development was not caused by fertilization problems but more likely by intrinsic oocyte factors. Similar developmental rates achieved through nuclear transfer and parthenogenetic activation suggests that the nucleus of the incompetent oocyte may not be the sole reason for a poor development. Another important factor appears to be the donor animal age. The younger the donor animal, the more impaired is its oocyte's developmental competence in most of the embryo IVP systems. Treatment with exogeneous gonadotropins can be beneficial in young donors on the oocyte cleavage rates but does not always increase the final blastocyst outcome. This review briefly documents some of the biological factors and their possible effects on the developmental capacities of the bovine oocyte in vitro.